JP7638480B2 - Foundation structure and its construction method - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. Date of application for evaluation: May 14, 2021. Date of publication (evaluation date): July 9, 2021. Location of publication (website address): https://www.bcj.or.jp/db/. Publisher: The Construction Center of Japan, published content: "Evaluation Report (Construction Method, etc.) (BCJ Evaluation-FD0141-06), The Construction Center of Japan, July 9, 2021." [Publications, etc.] Date of website posting: July 9, 2021. Website address: http://www.ftpile.jp/working_rooms/shiryo.htm. Publisher: F. T. Pile Construction Method Precast Pile Association, Public Contents "Evaluation Report (Construction Method, etc.) (BCJ Evaluation-FD0141-06), The Building Center of Japan, July 9, 2021"

The present invention relates to a foundation structure in which a structure foundation is placed on the pile heads of prefabricated piles installed in the ground, and a method for constructing the same.

Aoshima et al., one of the inventors of the present invention, have developed the following construction method as a construction method for a so-called semi-rigid joint type foundation structure (a foundation structure in which a structure foundation is placed on the pile head of a prefabricated pile installed in the ground, hereinafter sometimes simply referred to as a "semi-rigid joint structure") that can reduce bending moments occurring at the pile head while ensuring high shear force transmission capacity during an earthquake. That is, this construction method for a semi-rigid joint structure includes the steps of embedding a prefabricated pile, the pile head of which is provided with a jig that engages with a rotary press-in device, in the ground using the rotary press-in device, removing the jig, covering the pile head with a formwork with an inclined inner surface to form a gap that expands downward between the side of the pile head and the inner surface of the formwork, pouring leveling concrete around the formwork, and pouring foundation concrete that will become the structure foundation on the hardened leveling concrete (Patent Document 1).

JP 2007-23602 A

However, with foundation structures constructed using conventional construction methods, when horizontal forces are applied during earthquakes or strong winds, etc. (hereinafter referred to as "earthquakes, etc."), shear failure occurs at the joints between the underside of the structure's foundation and the heads of the prefabricated piles, and punching shear failure can occur due to the application of localized concentrated loads, so measures to address this issue were required.

The present invention was made to solve the above problems, and aims to provide a foundation structure and a method for constructing the same that can prevent shear failure at the joint between the underside of the structure foundation and the head of the precast pile, assuming a semi-rigid joint type foundation structure.

In order to solve the above problems, the present invention provides a foundation structure in which a reinforced concrete structural foundation having a recess on its underside is placed on the heads of prefabricated piles driven into the ground in the recess, the recess being formed wider downward, and a formwork having a flange on its periphery that is integral with the recess is provided, and a cage-shaped restraining bar is provided to surround the upper surface of the formwork, the restraining bar comprising a plurality of vertical bars and horizontal bars surrounding the plurality of vertical bars, and each of the vertical bars is arranged to extend in the vertical direction of the lowermost end bar of the structural foundation (hereinafter referred to as "this foundation structure").

Here, "placing" refers to a method of constructing a foundation structure in which a structural foundation (regardless of the shape or form of the structural foundation) is placed on the head of a precast pile that has been driven (buried) into the ground, and in order to support the structural foundation, the structural foundation is simply placed on the head of the precast pile, and is in a semi-rigid connection state in which the degree of fixation of the joint is reduced to allow rotation.
The explanations of the terms mentioned above and others are common to the present basic structure and the method of constructing the present basic structure described below.

The formwork must be frustum-shaped (the inner periphery is inclined so that it becomes wider toward the bottom), have a flange formed on the periphery (lower edge) of the bottom surface, and have a space inside. It is preferable that the formwork be shaped like a truncated cone with an upper surface, have an open bottom surface, have an inner space, and have a flange formed on the edge of the circumference of the bottom surface. There are no restrictions on the dimensions and materials of the formwork (including the flange), but it is preferable that it be made of metal such as steel so that it can withstand the lateral pressure of the concrete and prevent damage.

The restraining bars are also provided to resist the shear force that occurs near the recess of the structure foundation (near the pile head joint) when a horizontal force is applied. Therefore, they need to be appropriately provided so as to surround the vicinity of the top surface of the formwork. Therefore, it is necessary to arrange vertical bars (anchoring bars arranged vertically) and horizontal bars (transverse reinforcing bars arranged horizontally) to achieve the above-mentioned effect, and the number, specifications, weight, installation style, etc. of the bars can be appropriately determined.
The horizontal reinforcement is a transverse reinforcement formed by bending steel bars into a closed shape, and it is preferable to use spiral reinforcement and multiple ring-shaped hoop reinforcement.

In addition, the lowest bottom reinforcement in a structure foundation means the reinforcing bar that is placed at the bottom in the height direction when there are multiple reinforcing bars that make up the bottom reinforcement placed in the structure foundation.
With regard to the arrangement of vertical reinforcement, "arranged so as to extend above and below the bottom reinforcement of the lowest part of the structure's foundation" means that the upper end of the vertical reinforcement is arranged above the bottom reinforcement of the lowest part, and the lower end of the vertical reinforcement is arranged below the bottom reinforcement of the lowest part.

According to this foundation structure, a cage-shaped restraining bar is formed in the structure foundation so as to surround the upper surface of the formwork, and each vertical bar that makes up the restraining bar is arranged so as to extend in the vertical direction of the lowermost lower end bar in the structure foundation. This makes it possible to effectively prevent shear failure that occurs near the recess in the structure foundation (near the pile head joint) during an earthquake, etc.

The present invention also provides a method for constructing a foundation structure in which a reinforced concrete structure foundation is placed on the head of a prefabricated pile installed in the ground, the method comprising the steps of: a prefabricated pile driving step of driving the prefabricated pile into the ground; a formwork installation step of covering the head of the pile with a frustum -shaped formwork having a flange formed on its periphery (hereinafter, a formwork having a flange formed thereon may be referred to as a "flanged formwork") to form a gap between the side of the head of the pile and the inner peripheral surface of the formwork; a leveling concrete pouring step of pouring leveling concrete around the formwork; a constraining bar reinforcing bar arrangement step of arranging cage-shaped constraining bars so as to surround the upper surface of the formwork; and a structure foundation concrete pouring step of pouring foundation concrete for the structure foundation on the leveling concrete while the formwork is constrained by the leveling concrete (hereinafter, referred to as "this construction method").

In addition, in this construction method, the restraining reinforcement comprises multiple vertical reinforcements and horizontal reinforcements surrounding the multiple vertical reinforcements, and each of the vertical reinforcements is arranged to extend above and below the bottom end reinforcement of the lowest part of the structure foundation, which is preferable because it improves workability.

Here, ground material is a general term for crushed stone (gravel), sand, clay, soil, etc. used at the site.
In addition, when placing ground material around the formwork, an appropriate method can be used depending on the conditions of the target site, such as piling crushed stone or the like around the outer periphery of the formwork and burying the lower end of the formwork in the ground (floor surface).

In addition, there is no restriction on the method of driving the prefabricated piles into the ground during the prefabricated pile driving process. However, if a prefabricated pile with a jig that engages with the rotary driving device protruding from its head is buried in the ground using the rotary driving device, and the jig for the rotary driving device attached to the prefabricated pile is removed before the formwork is placed over the head of the prefabricated pile, noise during construction can be prevented and the problem of the jig restricting the rotation of the head of the prefabricated pile after driving can be eliminated, making this method preferable.

According to this construction method, since it includes a reinforcing bar arrangement process in which cage-shaped reinforcing bars are arranged to surround the upper surface of the formwork, it is possible to easily construct this foundation structure which can prevent shear failure.
In addition, the formwork used in the formwork installation process is frustum-shaped and has a flange formed around the periphery of its underside, making it easy to handle during construction, which helps to ensure safety during construction.

Furthermore, because the leveling concrete is poured using a formwork with a flange, the presence of the flange makes it difficult for the formwork to shift or deform when pouring the concrete for the structure's foundation, effectively preventing concrete and ground materials from entering the gaps.

In addition, in this construction method, if the formwork is fixed to the pile head using fasteners before the smoothing concrete pouring process and the fasteners are removed before the foundation concrete pouring process, this is preferable because it reliably prevents minute shifts in the formwork that may occur when the smoothing concrete is poured, and therefore makes it possible to more reliably obtain the joint condition envisaged at the time of design.

In addition, in this construction method, the outer peripheral surface of the formwork is marked along its lower edge, and if ground material or leveling concrete is placed on the outer periphery of the lower end of the formwork up to the position of the mark before the leveling concrete pouring process, this is convenient for construction and prevents concrete from flowing into the gap formed between the side of the pile head and the inner peripheral surface of the formwork, making it possible to reliably secure a gap around the pile head, which is preferable.

The present invention provides a foundation structure and a method for constructing the same that can prevent shear failure at the joint between a semi-rigid prefabricated pile and a foundation structure.

1A and 1B are side cross-sectional views showing a foundation structure of the present invention, in which FIG. 1A shows a normal state and FIG. FIG. 2A is a perspective view showing a precast pile used in the foundation structure of the present invention, and FIG. 2B is a perspective view showing a formwork used in the foundation structure of the present invention. 3A is a perspective view showing a cage-shaped restraining bar used in the foundation structure of the present invention, and FIG. 3B is an enlarged view of the X portion of FIG. 3A. FIG. FIG. 4 is a side cross-sectional view showing another embodiment of the foundation structure of the present invention. 1A to 1C are side cross-sectional views for explaining the prefabricated pile driving process in the foundation structure construction method of the present invention. 1A and 1B are side cross-sectional views for explaining a method for constructing a foundation structure of the present invention, in which (a) shows a formwork installation step and (b) shows a leveling concrete pouring step. A side cross-sectional view showing another embodiment of the formwork installation step in the foundation structure construction method of the present invention. 1A and 1B are side cross-sectional views for explaining a method for constructing a foundation structure of the present invention, in which FIG. 1A shows a constraining reinforcement step, and FIG. 1B shows a structure foundation concrete pouring step.

Below, an example of an embodiment of this foundation structure and this construction method will be described in detail with reference to the drawings. In the description based on the drawings, the same elements are given the same reference numerals and duplicated explanations will be omitted. Also, in Figures 1(a), 1(b), 4, 8(a) and 8(b), the cage-shaped restraining bars 40 are illustrated in a simplified form for drawing purposes.

[Basic structure]
First, before explaining the construction method, the basic structure K will be explained.
This foundation structure K is a semi-rigid joint type foundation structure that reduces the bending moment applied to the pile head while ensuring shear force transmission capacity, and is a structure in which a pile cap 2, which is the foundation of a reinforced concrete structure, is placed on the pile head of a prefabricated pile 1 driven into the ground G (Figure 1 (a)).

(Prefabricated piles)
The prefabricated pile 1 comprises a cylindrical body 11 having a hollow interior, and an annular end plate 12 attached to the upper end surface of the body 11 (FIG. 2(a)).
In this embodiment, the main body 11 has PC steel bars (not shown) arranged in the longitudinal direction (vertical direction).
Furthermore, the material of the main body 11 is not important as long as it can obtain the required strength, and it can be made of prestressed concrete, concrete with an outer steel pipe and steel pipe, etc.

The end plate 12 is made of steel, and is fixed to the upper end surface of the main body 11 by embedding an anchor (not shown) protruding from the bottom surface into the main body 11. The end plate 12 also has a plurality of female screw holes 12a formed at predetermined intervals in the circumferential direction (FIG. 2(a) shows a jig 13 used in the prefabricated pile driving step of this construction method).
In addition, when the main body 11 is made of steel, the end plates 12 are usually fixed by welding, but in some cases, the end plates 12 can be omitted.

(Pile cap)
The pile cap 2 is a structure made of reinforced concrete, and has a truncated cone-shaped recess 21 (a recess that widens downward) formed on its bottom surface. That is, the formwork 3 is placed over the top of the prefabricated pile 1, and then the foundation concrete that will become the pile cap 2 is poured. With the above-mentioned configuration, the formwork 3 and the foundation concrete are integrated, and a recess 21 that is the same shape as the space in the formwork 3 is formed.

The formwork 3 has a disk-shaped upper surface 31 that abuts against the upper end surface (upper surface of the end plate 12) of the precast pile 1, whose inner periphery is inclined, and a side portion 32 that surrounds the head of the precast pile 1, and has an external appearance of a truncated cone (Figure 2 (b)).

The upper surface portion 31 has the same planar shape as the upper end surface of the pile head of the prefabricated pile 1. The upper surface portion 31 has through holes 31a for temporarily fixing the end plate 12 formed in a plurality of locations (two locations in this embodiment) corresponding to the female screw holes 12a of the end plate 12.
The side portion 32 is provided on the outer periphery of the upper surface portion 31, and its inner diameter gradually increases downward, with a flange portion 32b formed on the periphery of the lower surface.

Furthermore, a mark 32 a is provided on the outer peripheral surface of the side portion 32 along the lower edge of the formwork 3 to serve as a guide when embedding the side portion 32 into the crushed stone layer S.
It is preferable that the mark 32a is formed at a position at least 2 cm above the lower edge of the formwork 3. In the present embodiment, the mark 32a is marked so as to be continuous around the entire circumference of the side portion 32, but this is not limiting and the mark may be marked discontinuously.

The head of the precast pile 1 is placed in contact with the inner surface of the upper surface 31 of the formwork 3, and the presence of the formwork 3 forms a gap E around the head of the precast pile 1.

A crushed stone layer S of a predetermined thickness is formed on the ground G, and a leveling concrete layer C of a predetermined thickness is provided on top of the crushed stone layer S, with the position of the mark 32a on the side portion 32 as its bottom surface. However, depending on the ground conditions, etc., it is not necessary to provide a crushed stone layer S.

The pile cap 2 is constructed on the top surface of the leveled concrete layer C. The pile cap 2 has multiple upper end reinforcements 22 arranged horizontally at a predetermined position a predetermined height below the top surface. Additionally, multiple lower end reinforcements 23 are arranged horizontally at a predetermined position a predetermined height above the bottom surface (upper surface 31 of the formwork 3). Reference numeral 24 denotes stirrups arranged around the upper end reinforcements 22 and lower end reinforcements 23.

The cage-shaped restraining bars 40 are arranged to surround the upper parts of the top surface 31 and side surfaces 32 of the formwork 3. The vertical bars 41 constituting the cage-shaped restraining bars 40 are spaced a predetermined distance from the top surface 31 and side surfaces 32 of the formwork 3 to ensure a sufficient cover thickness.

The cage-shaped restraining bar 40 has a plurality of vertical bars 41 (eight in this embodiment) arranged at regular intervals according to the number of bars, a pair of two ring members 43, one above the other, surrounding the vertical bars 41, and a spiral bar 42 (closed horizontal bar) arranged between the upper and lower ring members 43 (Figures 3(a) and (b)).
The thin steel ring member 43 is a member that is installed horizontally to ensure the accuracy of the placement of the vertical bars 41, and two mounting holes 43a for U-bolts 46 are provided at each mounting position of the vertical bars 41.

The vertical reinforcements 41 are installed at a predetermined interval, and are attached to the upper and lower ring members 43 by U-bolts 46 and nuts 47 in the mounting holes 43a with the inner side of each vertical reinforcement 41 abutting the outer side of each ring member 43. The spiral reinforcements 42 are also arranged between the upper and lower ring members 43 at a predetermined interval in the height direction.

The upper surface 31 and the upper part of the side 32 of the formwork 3 are inserted into the central space surrounded by each vertical reinforcement 41 of the cage-shaped restraining reinforcement 40. The upper end of each vertical reinforcement 41 is arranged near the upper end reinforcement 22, and the lower end is arranged near the bottom surface of the pile cap 2 (below the lower end reinforcement 23), and extends in the vertical direction of the lower end reinforcement 23.

The vertical spacing between the upper and lower ring members 43 must be within the range of reinforcement provided by the spiral reinforcement 42 (preferably within a range of ±10 cm from the head of the prefabricated pile 1). The spiral reinforcement 42 is attached with tie wires 48 at the points where it intersects with each vertical reinforcement 41.

(Action and Effect)
According to this foundation structure K, due to the presence of the formwork 3 covering the head of the precast pile 1, a gap E exists around the head of the pile. This allows the head of the pile to rotate during an earthquake, etc., and has the effect of preventing a large bending moment from being generated at the head of the pile even if a horizontal force Q (shear force) acting on the structure acts on the precast pile 1 (Figure 1(b)).

In addition, according to this foundation structure K, a cage-shaped restraint reinforcement 40 is provided in the pile cap 2 so as to surround the upper surface of the formwork 3, and the vertical reinforcement 41 has its upper end arranged near the upper end reinforcement 22 and its lower end arranged below the lower end reinforcement 23, thereby effectively preventing shear failure occurring near the recess 21 in the pile cap 2 during an earthquake, etc.

(Modification)
The foundation structure K of the above embodiment can be suitably used in cases where no pull-out force acts on the prefabricated pile 1.
On the other hand, when a pull-out force acts on the precast pile 1, it is possible to effectively deal with the pull-out force by adopting a structure in which a predetermined number of pull-out resistance rods 14 are provided in the pile cap 2' in the vertical direction so that the length of the rods is approximately the same as that of the vertical reinforcement 41 (hereinafter referred to as "modified foundation structure K'"). The pull-out resistance rods 14 are inserted into circular holes 31b provided in the upper surface 31 of the formwork 3 and fixed to the end plate 12 of the precast pile 1 (Figure 4).

The other structures in the modified basic structure K' are almost the same as those of the basic structure K described above, and the same effects as those of the basic structure K can be achieved.
In addition, the pull-out resistance bars 14 can be made of, for example, PC steel bars or rolled steel bars for construction structures (SNR), but the number and strength of the bars are set to an extent that a semi-rigid connection can be maintained, and there are no restrictions on the method of attachment to the end plates 12 .

[This construction method]
Next, the construction method of this embodiment will be described in detail.
This construction method is composed of processes including: (1) driving prefabricated piles; (2) installing formwork; (3) pouring leveling concrete; (4) reinforcing bars; and (5) pouring concrete for the structure foundation.

(1) Prefabricated pile driving process The prefabricated pile driving process is a process for driving a prefabricated pile into the ground G. In this embodiment, a prefabricated pile 1 having a jig 13 that engages with a rotary driving device protruding from the pile head is driven into the ground G using a rotary driving device (not shown).

The jig 13 engages with the drive shaft of the rotary driving device and is provided to prevent twisting of the prefabricated pile 1 during rotary driving. It is made of steel material with an inverted L-shape and is fixed to the end plate 12 by welding (FIG. 2(a)).
The jig 13 may be detachably fixed to the end plate 12 by using a mechanical joining method. In addition, when the main body 11 of the prefabricated pile 1 is formed of a steel pipe or concrete with an outer steel pipe, the jig 13 may be fixed to the side of the upper end of the main body 11 by welding or the like.

In this process, a rotary driving device is used to drive the prefabricated pile 1 into the ground G (Figure 5(a)). In addition, to prevent soil and sand from clogging the female threaded hole 12a of the end plate 12 when rotary driving the prefabricated pile 1, a bolt B1 is screwed into the female threaded hole 12a in advance.

After the prefabricated pile 1 is driven, the surrounding ground G is dug down to expose the pile head (Fig. 5(b)). Then, the base of the jig 13 attached to the end plate 12 of the prefabricated pile 1 is removed by an appropriate method such as melting, and the cross section is polished with a grinder or the like to smooth the top surface of the prefabricated pile 1, and the bolt B1 screwed into the female threaded hole 12a is removed (Fig. 5(c)).

Furthermore, before and after removing the jig 13, any unevenness in the bedding surface G1 is corrected, and then crushed stone is laid and compacted on the bedding surface G1 to form a crushed stone layer S.
In addition, if the crushed stone layer S is not provided due to ground conditions, etc., the process of forming the crushed stone layer S is omitted.

(2) Formwork installation process The formwork installation process is a process in which the formwork 3 is placed over the head of the precast pile 1, a gap E is formed between the side of the head of the pile and the inner surface of the side portion 32 of the formwork 3, the gap E expanding downward, and ground material S' is placed on the outer periphery of the lower end of the formwork 3.

In this process, with the upper surface 31 of the formwork 3 in close contact with the upper end surface of the precast pile 1, the formwork 3 is placed over the head of the precast pile 1, and a gap E is formed between the side surface 11a of the head of the pile and the inner surface of the side portion 32 of the formwork 3, the diameter of which increases as it extends downward.
When covering the end plate 12 of the pile head with the formwork 3, it is necessary to align the positions of the through hole 31a of the formwork 3 and the female screw hole 12a of the end plate 12 in advance (Figures 2(a) and (b)).

Next, the pile head of the prefabricated pile 1 is covered with the formwork 3, and then the formwork 3 is temporarily fixed to the pile head using a bolt (fastener) B2 that screws into the female threaded hole 12a.
In addition, without providing a female screw hole 12a in the end plate 12, the formwork 3 can be fixed to the pile head by adhesive, welding, etc., with a strength that does not reduce the rotation performance of the pile head.

After the formwork 3 is temporarily fixed to the head of the precast pile 1, the mark 32a on the outer periphery is used as a guide to fill the outer periphery of the lower end of the formwork 3 with soil material S', thereby closing the gap between the lower edge of the formwork 3 and the top surface of the crushed stone layer S (Figure 6 (a)).

Depending on the conditions at the site, the lower end of the formwork 3 can be embedded into the crushed stone layer S (corresponding to ground material S') up to the position of the mark 32a, thereby placing ground material S' around the lower end of the formwork 3 and the outer periphery of the flange portion 32b (not shown).
Alternatively, this can be achieved by digging down the floor surface G1 of the ground G and then placing ground material S' such as crushed stone or soil around its periphery (Figure 7 shows another embodiment of this process), or by burying the lower end and flange portion 32b of the formwork 3 in the ground G and then forming a crushed stone layer S (corresponding to ground material S') on the floor surface G1 (Figure 4).

In this embodiment, the head of the precast pile 1 is covered with the formwork 3 before the crushed stone layer S is formed, but the head of the precast pile 1 may also be covered with the formwork 3 after the crushed stone layer S is formed.
In addition, in this embodiment, the formwork 3 is fixed to the pile head using the bolts B2 before the ground material S' is piled around the outer periphery of the formwork 3, but it may be fixed after the ground material S' is piled.

(3) Leveling Concrete Pouring Process The concrete pouring process is a process for pouring leveling concrete around the formwork 3.

In this process, after placing a ground material S' around the outer periphery of the lower end of the formwork 3, leveling concrete is poured around the formwork 3 to form a leveling concrete layer C (Figure 6 (b)).
Although there is no limit to the thickness of the leveling concrete layer C, it is preferable to set the height from the top surface of the leveling concrete layer C to the top surface of the formwork 3 to 5 cm or more.

After the leveling concrete has hardened, remove the bolt B2. At this time, tap the top surface 31 of the formwork 3 with a mallet or the like to check again that the top surface 31 is in close contact with the top surface of the prefabricated pile 1, and tap the side surface 32 with a mallet or the like to check that the leveling concrete has not flowed into the gap E.

(4) Constraining Bar Arrangement Process The constraining bar arrangement process is a process in which the various reinforcing bars (reinforcing bars necessary for foundations such as mats and footings, including upper end bars 22, lower end bars 23 and stirrup bars 24) to be installed in the pile cap 2 are arranged, and a cage-shaped constraining bar 40 fabricated in advance is arranged to surround the upper surface of the formwork 3 (Figure 8 (a)).

In this process, in addition to the above-mentioned rebars, cage-shaped restraining bars 40 (see the explanation in the present foundation structure K) are arranged. At this time, the upper parts of the upper surface part 31 and the side part 32 of the formwork 3 are inserted into the central space surrounded by the vertical bars 41 of the cage-shaped restraining bars 40, and the cage-shaped restraining bars 40 are installed so as to surround the formwork 3. In addition, the vertical bars 41 are spaced apart so as to secure a predetermined distance between the upper surface part 31 and the side part 32 of the formwork 3, and the upper ends of the vertical bars 41 are arranged near the upper end bars 22 and the lower ends are arranged near the upper surface (the bottom surface of the pile cap 2) of the flat concrete layer C (below the lower end bars 23), so that the vertical bars 41 extend in the vertical direction of the lower end bars 23.
In addition, spacers (not shown) must be interposed in the vertical and horizontal directions in at least three or more places between the formwork 3 and the cage-shaped restraint bars 40 so that the cage-shaped restraint bars 40 are evenly spaced around the periphery of the formwork 3.

(5) Structure Foundation Concrete Pouring Process The structure foundation concrete pouring process is a process in which, while the formwork 3 is restrained by the hardened leveled concrete layer C, foundation concrete that will become the pile cap 2 is poured on the leveled concrete layer C to construct the pile cap 2.

In this process, the reinforcing bars and formwork (not shown) of the pile cap 2 are placed on top of the smoothed concrete layer C, and foundation concrete is poured on top of the smoothed concrete layer C and allowed to harden, completing the construction of the foundation structure K (Figure 8 (b)).

(Action and Effect)
According to this construction method, a constraining bar arrangement process is included in which prefabricated cage-shaped constraining bars 40 are arranged to surround the upper part of the upper surface 31 and the upper part of the side portion 32 of the formwork 3, so that the foundation structure K capable of preventing shear failure can be easily constructed.
In addition, the formwork 3 used in the formwork installation process has a truncated cone shape and is formed with a flange portion 32b on its lower edge, making it easy to handle during construction, thereby helping to ensure safety during construction.
Furthermore, when pouring the concrete of the pile cap 2, the formwork 3 is less likely to shift or deform, and it is possible to effectively prevent the concrete and ground material S' from entering the gap E. Note that the ground material S' arranged around the bottom end (brim) of the formwork 3 seals the periphery of the bottom end of the formwork 3, so that it is possible to prevent the leveling concrete from flowing into the gap E.

Furthermore, by adopting this construction method, it is possible to realize a semi-rigid connection state between the prefabricated pile 1 and the pile cap 2 as envisaged at the time of design. In other words, since the jig 13 for the rotary press-in device provided on the prefabricated pile 1 is removed before the formwork 3 is placed over the pile head of the prefabricated pile 1, it is possible to eliminate the problem of the rotation of the pile head being restricted by the jig 13.
Furthermore, since the formwork 3 can be restrained by the leveling concrete layer C, the formwork 3 is less likely to shift or deform when the concrete of the pile cap 2 is poured, and further, it is possible to prevent foundation concrete from seeping into the gap E around the pile head formed by the formwork 3.

Before the leveling concrete is poured, the formwork 3 is fixed to the head of the prefabricated pile 1 using bolts B2, which reliably prevents minute shifts in the formwork 3 that may occur when the leveling concrete is poured, and as a result, the joint condition envisaged in the design can be more reliably achieved. Furthermore, because bolts B2 are removed before the foundation concrete is poured, the rotation of the pile head is not restricted, and freedom of rotation is ensured.

While one example of a preferred embodiment of the present invention has been described above, the present invention is not limited to this embodiment, and appropriate design changes can be made to each element without departing from the spirit of the present invention.
There are no limitations on the form of precast piles and the foundation of the structure, or on the construction site of the present invention, and the optimal components can be adopted for each component of the present invention. In addition, the above invention-specific matters specify the minimum necessary components, and other components may be added as long as they do not impair the effects of the invention.

K, K' Foundation structure G Ground S' Ground material S Crushed stone layer C Leveled concrete layer E Gap 1 Prefabricated pile 2 Pile cap 3 Formwork 11 Main body 21 Recess 22 Upper end bar 23 Lower end bar 24 Stirrup bar 31 Upper surface 32 Side 32a Mark 32b Flange 40 Cage-shaped restraining bar 41 Vertical bar 42 Spiral bar 43 Ring member

Claims (3)

A method for constructing a foundation structure in which a reinforced concrete structure foundation is placed on the head of a precast pile installed in the ground, comprising the steps of:
A prefabricated pile driving process of driving the prefabricated pile into the ground;
A formwork installation process in which a frustum -shaped formwork having a flange formed on the periphery is placed over the pile head to form a gap between the side surface of the pile head and the inner peripheral surface of the formwork;
A leveling concrete pouring process of pouring leveling concrete around the formwork;
A constraining bar arrangement process of arranging a cage-shaped constraining bar so as to surround the upper surface portion of the formwork;
A method for constructing a foundation structure, comprising: a structure foundation concrete pouring step of pouring foundation concrete for the structure foundation on the leveling concrete while restraining the formwork with the leveling concrete. The restraining reinforcement includes a plurality of vertical reinforcements,
and horizontal bars surrounding the plurality of vertical bars;
2. The method for constructing a foundation structure according to claim 1, wherein each of the vertical reinforcements is arranged so as to extend in the vertical direction of the lowermost end reinforcement of the structural foundation. A reinforced concrete structural foundation having a recess on the underside,
A foundation structure placed on a pile head of a prefabricated pile driven into the ground in the recess,
The recess is formed to be wider downward,
A mold having a flange portion on a periphery is provided integrally with the recess,
A cage-shaped restraining bar is provided to surround the upper surface of the formwork,
The restraining reinforcement includes a plurality of vertical reinforcements,
and a horizontal bar surrounding the plurality of vertical bars;
A foundation structure characterized in that each of the vertical reinforcements is arranged so as to extend in the vertical direction of the lowermost end reinforcement of the structural foundation.